Heat-resistant maleimido polymers/block copolymers

ABSTRACT

Novel N-substituted maleimide homopolymers having a number-average molecular weight ranging from 12,000 to about 100,000 are prepared by anionically polymerizing an N-substituted maleimide monomer, in solvent phase, in the presence of at least one organometallic polymerization initiator which comprises sec-butyllithium, a sodium alcoholate, diphenylmethylpotassium, naphthalenelithium or naphthalenesodium, and novel N-substituted maleimide homopolymers having a polydispersity index ranging from 1.1 to 1.5 are prepared by anionically polymerizing an N-substituted maleimide monomer, in solvent phase, in the presence of at least one alkali metal organometallic polymerization initiator and at least one inorganic or organic salt of an alkali or alkaline earth metal; novel block copolymers include blocks of the above maleimido homopolymers and, e.g., blocks of an acrylic or methacrylic polymer.

This application is a continuation of application Ser. No. 07/618,637,filed Nov. 27, 1990 now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to novel polymers having improvedresistance to heat, and, more especially, to novel polymers comprisingmaleimido recurring structural units.

2. Description of the Prior Art

The polymerization of N-substituted maleimides by free radical andanionic routes is known to this art. Thus, R, Cubbon, Polymer, 6, 419(1965) has reported that polymers having a predominantlythreodiisotactic configuration are obtained by polymerizingN-ethylmaleimide using n-butyllithium as initiator:

(i) either in toluene, at temperatures not exceeding -20° C., for periodof time ranging from 20 minutes to 3 hours, providing a yield notexceeding 22%; or

(ii) in tetrahydrofuran, at -70° C., rapidly providing a yield rangingup to 95%.

With regard to N-phenylmaleimide, T. Hagiwara et al, Makromol. Chem.,Rapid Commun., 6, 169 (1985) has described the influence of theselection of the initiator and the reaction conditions on the anionicpolymerization and has confirmed the observations of R. Cubbon and alsoshown that:

(i) yields in excess of 85% are obtained in tetrahydrofuran between -72°C. and O° C. for reaction periods of 30 to 60 minutes in the presence ofpotassium tert-butanolate. The number-average molecular weight of thepolymer obtained can be as high as 8,000 for a reaction period of 3hours at -72° C.; and

(ii) a quantitative yield in the presence of a large proportion oflithium tert-butanolate can be obtained at O° C. in tetrahydrofuran, theresulting polymer having a number-average molecular weight of only2,000.

As regards N-ethylmaleimide, the above observations have also beenconfirmed by T. Hagiwara et al, in J. Polym. Sci., Polym. Chem. Ed., 26,1011 (1988). However, in addition to the above confirmation, the authorspresent the possibility of excellent polymerization yields at +24° C. inthe presence of potassium tert-butanolate, equally well in toluene as intetrahydrofuran, the active nature of the reaction process alsopermitting a number-average molecular weight of up to 6,400 to beattained.

From the above prior art, it will be seen, in summary, that, on the onehand, the polymerization of N-substituted maleimides by an anionic routeproduces results which are highly differentiated depending on thenature, alkyl or aryl, of the N-substitution and, with a singleexception, depending on the nature of the solvent used. On the otherhand, to date no process has been proposed to the art enabling polymersof high molecular weight to be produced (the attainable degree ofpolymerization appearing to be limited to about 50). From the firstobservation, it will be appreciated that need exists in this art forpolymers having a controlled, and preferably narrow molecular weightdistribution, prepared by a process that proceeds under industriallysafe conditions. From the second observation, it will be appreciatedthat polymers having too low a molecular weight have scant industrialapplications.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofimproved technique for synthesizing, in good yields, novel N-substitutedmaleimide polymers having a narrow molecular weight distribution, saidnovel polymers comprising recurring structural units derived frommaleimides having intrinsic characteristics corresponding to the actualindustrial applications intended therefor, and in particular polymers ofhigh molecular weight.

Another object of the present invention is the provision of such novelpolymers, i.e., polymers comprising blocks of high molecular weightbased on N-substituted maleimide, using specific anionic polymerizationinitiators not heretofore employed in the polymerization of this type ofmonomer. Thus, a judicious selection of these initiators enables polymerblocks to be obtained having a molecular weight which is at least equalto 12,000, namely, half as high again as the highest molecular weightknown to date, and can be up to about 100,000.

It has also now been determined that, whatever the anionicpolymerization initiator used and, thus, whatever the molecular weightof the base N-substituted maleimide polymer, a narrow distribution ofmolecular weights can be attained by adding a compound selected fromamong the inorganic salts and the organic salts of alkali metals oralkaline earth metals to the polymerization initiator. The addition ofsuch a compound generally enables the polydispersity of the polymerblock based on N-substituted maleimide to be lowered to a value rangingfrom approximately 1.1 to 1.5, although, in the absence of suchadditive, the polydispersity is currently in excess of 1.7. As is wellknown to the art of anionic polymerization, such a difference inpolydispersity most typically presents the advantage of favorablymodifying the characteristics and properties of the products which canbe produced from these polymers.

Finally, it has also now been found that polymer blocks based onN-substituted maleimide, if necessary of high molecular weight and/or oflow dispersity as described above, can advantageously be combined withpolymer blocks based on acrylic or methacrylic monomers, for example inthe form of bi-block or tri-block copolymers.

Briefly, the present invention features a process for the anionicpolymerization of an N-substituted maleimide in a solvent and in thepresence of at least one alkali metal organometallic initiator, whereinthe polymerization initiator used is a compound selected from amongsec-butyllithium, sodium alcoholates, diphenylmethylpotassium,naphthalenelithium and naphthalenesodium.

As indicated above, the principal effect of the process according to theinvention is provided by the increase in the molecular weight of theresulting polymer. Indeed, the process according to the inventionpermits the preparation of polymer blocks, based on N-substitutedmaleimide, having a molecular weight which is at least equal to 12,000and which can range up to about 100,000, even though known anionicpolymerization initiators, such as either those already proposed to theart in respect of maleimides (n-butyllithium, potassium tert-butanolate,lithium tert-butanolate) or others such as cumylpotassium,tert-butyl-lithium or alpha-methylstyryllithium, have been shown to beincapable of preparing N-substituted polymaleimides having a molecularweight in excess of 8,000.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

More particularly according to the present invention, by "N-substitutedmaleimide" is intended a monomer of the formula: ##STR1## in which R isan alkyl, arylalkyl, aryl or alkylaryl radical having from 1 to 12carbon atoms. Exemplary such monomers include, in particular,N-ethylmaleimide, N-isopropylmaleimide, N-n-butylmaleimide,N-isobutylmaleimide, N-tertbutylmaleimide, N-n-octylmaleimide,N-cyclohexylmaleimide, N-benzylmaleimide and N-phenylmaleimide.

In the process according to this invention, the solvent is preferablyselected from among aromatic solvents, such as benzene and toluene, ortetrahydrofuran, diglyme, tetraglyme, orthoterphenyl, biphenyl, decalin,tetralin or dimethylformamide or mixtures thereof, and the temperaturepreferably ranges from approximately -78° C. to +20° C.

The present invention also features a process for the preparation ofpolymers, based on N-substituted maleimide, having a narrow molecularweight distribution, by anionic polymerization of said maleimide in asolvent and in the presence of an initiator system comprising at leastone alkali metal organometallic initiator and at least one compoundselected from among the inorganic salts and the organic salts of alkalimetals or alkaline earth metals. Exemplary such organometallicinitiators include, in addition to those indicated above, compounds suchas diphenylmethyllithium, diphenylmethylsodium,1,4-dilithio-1,1,4,4-tetraphenylbutane and1,4-disodio-1,1,4,4-tetraphenylbutane.

The compound combined with the organometallic initiator is selectedfrom, on the one hand, inorganic salts of alkali metals or alkalineearth metals, for example the chlorides, fluorides, bromides, iodides,borides, sulfates, nitrates and borates, and, on the other, the organicsalts of alkali metals, for example the alcoholates, the esters ofcarboxylic acids substituted in the alpha-position by said metal and thecompounds in which the said alkali metal is associated with a group suchas:

(A) those of the formula: ##STR2## in which R₁ is a straight or branchedchain alkyl radical having from 1 to 20 carbon atoms, a cycloalkylradical having from 3 to 20 carbon atoms, or an aryl radical having from6 to 14 carbon atoms;

(B) those of the formula: ##STR3## in which Y and Z, which may beidentical or different, are each a hydrogen atom or a halogen atom, n isan integer ranging from 0 to 4, X is a halogen atom, and m is an integerranging from 0 to 2;

(C) those of the formula:

    --O--SO.sub.2 --CT.sub.3                                   (IV)

in which T is a hydrogen atom or a halogen atom; and

(D) those of the formula:

    B(R.sub.2).sub.4                                           (V)

in which R₂ is a hydrogen atom or an alkyl or aryl radical.

Exemplary of the groups of formula (II) are the acetate, propionate andbenzoate groups.

Exemplary of the groups of formula (III) are the alpha-bromoacetate andtrifluoroacetate groups.

Exemplary of the groups of formula (IV) are the trifluoromethanesulfonicand methanesulfonic groups.

Exemplary of the groups (V) are the borohydride and tetraphenylboridegroups.

In this "second" process according to the invention, the solvent ispreferably selected from among the aromatic solvents, such as benzeneand toluene, or tetrahydrofuran, diglyme, tetraglyme, ortho-terphenyl,biphenyl, decalin, tetralin or dimethylformamide or mixtures thereof,and the temperature preferably ranges from approximately -78° C. to +20°C. Of course, when the polymerization initiator is selected from amongsec-butyllithium, sodium alcoholates, diphenylmethylpotassium,naphthalenesodium and naphthalenelithium, the characteristic features ofabove two processes can be combined, which permits polymers based on anN-substituted maleimide to be produced which have, at one and the sametime, a high number-average molecular weight and a narrow molecularweight distribution.

The present invention also features a multi-block copolymer in which atleast one polymer block based on an N-substituted maleimide is combinedwith at least one polymer block based on an acrylic or methacrylicmonomer, with the exception of tri-block copolymers having anumber-average molecular weight ranging from 3,000 to 300,000 and apolydispersity of the molecular weights ranging from 1.05 to 2.0, andhaving at least one end block based on an N-substituted maleimide.

By "acrylic monomer" is intended a monomer selected from among primary,secondary or tertiary alkyl acrylates, in which the alkyl group, whichmay be substituted, for example by at least one halogen atom, such aschlorine or fluorine, and/or at least one hydroxyl group, has from 1 to18 carbon atoms. Exemplary such acrylic monomers include, in particular,ethyl acrylate, propyl acrylate, isopropyl acrylate, butyl acrylate,isobutyl acrylate, hexyl acrylate, tert-butyl acrylate, 2-ethylhexylacrylate, nonyl acrylate, lauryl acrylate, stearyl acrylate, cyclohexylacrylate, isodecyl acrylate and also phenyl acrylate, isobornylacrylate, the alkylthioalkyl or alkoxyalkyl acrylates, acrylonitrile anddialkylacrylamides.

By "methacrylic monomer" is intended a monomer selected from among alkylmethacrylates, in which the alkyl radical, which may be substituted, forexample by at least one halogen atom, such as chlorine or fluorine,and/or at least one hydroxyl group, has from 1 to 18 carbon atoms.Exemplary such methacrylic monomers include methyl, ethyl2,2,2-trifluoroethyl, n-propyl, isopropyl, n-butyl, sec-butyl,tert-butyl, n-amyl, i-amyl, hexyl, 2-ethylhexyl, cyclohexyl, octyl,i-octyl, decyl, beta-hydroxyethyl, hydroxypropyl and hydroxybutylmethacrylates and also glycidyl methacrylate, norbornyl methacrylate,methacrylonitrile and dialkylmethacrylamides.

The multi-block copolymers according to the invention can be, inparticular, bi-block copolymers in which a block based on N-substitutedmaleimide is bonded to a block based on an acrylic or methacrylicmonomer. They can also be tri-block copolymers in which a central blockbased on an N-substituted maleimide is bonded to two end blocks based onan acrylic or methacrylic monomer. It will of course be appreciatedthat, if the block based on an N-substituted maleimide has been preparedby the first process of the present invention, the resulting multi-blockcopolymer will comprise a block having recurring structural unitsderived from said maleimide and having a high molecular weight, forexample at least equal to 12,000 and even ranging up to approximately100,000. Similarly, if the block based on an N-substituted maleimide hasbeen prepared by the second process of the present invention, theresulting multi-block copolymer will comprise a block having recurringstructural units derived from said maleimide and having a lowpolydispersity of molecular weights, for example a polydispersityranging from approximately 1.1 to 1.5.

In the multi-block copolymers according to the invention, the averagemolecular weight of the block based on the acrylic or methacrylicmonomer can vary over wide limits, ranging, in particular, fromapproximately 2,000 to 100,000. This block can itself have a lowpolydispersity of molecular weights, if it has been prepared in thepresence of at least one compound selected from the inorganic salts andthe organic salts of alkali metals or alkaline earth metals as describedabove.

In one embodiment of the present invention, the blocks constituting anacrylic and/or methacrylic monomer block, as described above, can behydrolyzed to a corresponding acrylic acid and/or methacrylic acid blockand such block can, if desired, subsequently be saponified into acorresponding alkali metal acrylate and/or methacrylate block.

In another embodiment of the present invention, the blocks constitutingan acrylic and/or methacrylic monomer block, as described above, can betransesterified to another acrylic and/or methacrylic monomer block, forexample to replace a tertiary or secondary acrylate by a primaryacrylate.

The present invention also features a process for the production of abi-block copolymer in which a polymer block based on an N-substitutedmaleimide is bonded to a polymer block based on an acrylic ormethacrylic monomer, or of a tri-block copolymer in which the centralblock based on an N-substituted maleimide is bonded to two end blocksbased on an acrylic or methacrylic monomer, and which comprises:

(1) conducting an anionic polymerization of at least one acrylic ormethacrylic monomer using a monofunctional initiator and, if necessary,in the presence of at least one compound selected from among theinorganic salts and the organic salts of alkali metals or alkaline earthmetals and the non-nitrogenous macrocyclic complexing agents, such as toproduce a functional polymer block based on the acrylic or methacrylicmonomer; and

(2) next reacting such functional block thus produced with at least oneN-substituted maleimide to obtain a functional bi-block copolymercapable of reacting, in turn, with at least one acrylic or methacrylicmonomer to form a tri-block copolymer.

The monofunctional initiator which can be used in the above process canbe a compound of the formula:

    (R).sub.p --M                                              (I)

in which M is an alkali metal or alkaline earth metal (valency p of 1 or2); and R is a straight or branched chain alkyl radical having 2 to 6carbon atoms, or an alkyl radical having 1 to 6 carbon atoms substitutedby at least one phenyl group.

Exemplary such compounds include, in particular, sec-butyllithium,n-butyllithium and alpha-methylstyryllithium, 1,1-diphenylhexyllithium,diphenylmethyl-lithium or -sodium or -potassium and1,1-diphenyl-3-methylphenyllithium. The monofunctional initiator canalso be an alkali metal alcoholate.

The present invention also features a process for the preparation of atri-block copolymer in which a central block based on an acrylic ormethacrylic monomer is bonded to two end blocks based on anN-substituted maleimide, and which comprises:

(1) conducting an anionic polymerization of at least one acrylic ormethacrylic monomer using a bifunctional initiator and, if desired, inthe presence of at least one compound selected from among the inorganicsalts and the organic salts of alkali metals or alkaline earth metalsand non-nitrogenous macrocyclic complexing agents, as to produce adianion of the polymer block based on the acrylic or methacrylicmonomer; and

(2) next reacting such dianion with at least one N-substitutedmaleimide.

Exemplary bifunctional initiators which can be used in this process are,in particular, 1,4-dilithio-1,1,4,4-tetraphenylbutane,1,4-disodio-1,1,4,4-tetraphenylbutane, naphthalenesodium andnaphthalenelithium.

As in the "second" process of the present invention, the amount ofcompound which may be present during the synthesis of the multi-blockcopolymers can vary greatly with respect to the initiator. This amountcan be, for example, substantially in excess of the molar amount ofinitiator. This amount can also be less than or equal to the molaramount of initiator. Preferably, the ligand is introduced in a molarproportion with respect to the initiator ranging from approximately 0.3up to 15.

In the process according to the invention, the polymerizations of steps(1) and (2) preferably are carried out in the absence of moisture andoxygen and in the presence of at least one solvent, preferably selectedfrom among the aromatic solvents, such as benzene and toluene, ortetrahydrofuran, diglyme, tetraglyme, ortho-terphenyl, decalin, tetralinor dimethylformamide.

With respect to the polymerization or copolymerization temperature, thiscan range from about -78° C. to +20° C.

Moreover, the multi-block copolymers according to the invention can behydrolyzed at a temperature ranging from approximately 70° to 170° C.,under a pressure ranging from 1 to 15 bar and in the presence ofapproximately 0.5 to 10% by weight, relative to the copolymer, of anacid catalyst such as paratoluenesulfonic acid, methanetoluenesulfonicacid or hydrochloric acid, in a polar solvent, such as dioxane. Afterhydrolysis, the tri-block copolymers comprising acrylic and/ormethacrylic acid blocks can be precipitated in heptane, filtered off,washed to remove any trace amounts of catalyst and finally dried. Theycan also subsequently be neutralized using a methanolic potassiumhydroxide solution or tetramethylammonium hydroxide in solution in amixture of toluene and methanol to form the corresponding tri-blockionomers.

When the tri-block copolymers according to the invention comprise ablock derived from a tertiary or secondary alkyl acrylate, this blockcan also be transesterified in known manner to provide a primary alkylacrylate block.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

In said examples to follow, the exclusion chromatography was carried outusing a WATERS GPC 501 apparatus fitted with two linear columns, usingtetrahydrofuran as eluent at a flow rate of 1 ml/min. The number-averagemolecular weights were determined using an HP 502 membrane osmometer.

EXAMPLE 1

100 ml of pre-dried tetrahydrofuran and, with stirring, 10⁻⁴ mol ofsec-butyllithium were introduced into a pre-dried round-bottomed flaskunder a nitrogen atmosphere. The mixture was brought to a temperature of-78° C. using a mixture of acetone and solid carbon dioxide and asolution of 2 g of N-cyclohexylmaleimide in toluene was then added. Thesolvents and the monomer were purified by the customary techniques foranionic polymerization; in particular, the N-cyclohexylmaleimide wastreated successively with calcium hydride and triethylaluminum.

A poly(N-cyclohexylmaleimide) was thus obtained, in a yield of 100%,which had the following characteristics:

Mn=number-average molecular weight (in thousands);

Mw=weight-average molecular weight;

Mw/Mn=polydispersity of the molecular weights;

Tg=glass transition temperature (in degrees Celsius).

The values of these properties are reported in the Table below.

EXAMPLE 2

The procedure of Example 1 was repeated, but replacing sec-butyllithiumby diphenylmethylpotassium. The polymerization yield and thecharacteristics of the polymer obtained are reported in the table below.

EXAMPLE 3

The procedure of Example 1 was repeated, but replacingN-cyclohexylmaleimide by N-isopropylmaleimide. The polymerization yieldand the characteristics of the polymer obtained are reported in thetable below.

EXAMPLE 4

The procedure of Example 1 was repeated, but replacing sec-butyllithiumby sodium tert-butanolate. The polymerization yield and thecharacteristics of the polymer obtained are reported in the table below.

                  TABLE                                                           ______________________________________                                        Example   1       2          3     4                                          ______________________________________                                        Yield     100     100        100   85                                         -- Mn     32      24         20    80                                         -- Mw/-- Mn                                                                             2.1     2.3        2.0   1.7                                        Tg        280     284        n.d.  244                                        ______________________________________                                         n.d.: not determined.                                                    

EXAMPLE 5

Repeating the procedure of Example 1 and successively introducingtert-butyl acrylate and then N-cyclohexylmaleimide, the blockcopolymerization of these two monomers was carried out at -78° C. (thedurations of each step being, successively, 15 and 120 minutes) underthe following conditions:

Alpha-methylstyryllithium:0.43×10⁻³ mol

Lithium chloride:0.43×10⁻² mol

Tert-butyl acrylate:4.4 g

N-cyclohexylmaleimide:2.0 g

Tetrahydrofuran:50 ml.

A bi-block copolymer having the following characteristics:

Mn (acrylate)=10,600

Mn (maleimide)=3,900

Mw/Mn (total copolymer)=1.14

was obtained in a yield of 100%.

EXAMPLE 6

Repeating the procedure of Example 1 and successively introducing methylmethacrylate and then N-cyclohexylmaleimide, the block copolymerizationof these two monomers was carried out at -78° C. (the durations of eachstep being, successively, 15 minutes and 2 hours) under the followingconditions:

Diphenylmethyllithium:0.2×10⁻³ mol

Methyl methacrylate:0.9 g

N-cyclohexylmaleimide:4 g

Tetrahydrofuran:100 ml.

A bi-block copolymer having the following characteristics:

Mn (methacrylate)=6,100

Mn (maleimide)=2,300

Mw/Mn (total copolymer=1.35

was obtained in a yield of 40%.

EXAMPLE 7

The procedure of Example 6 was repeated, but changing the proportions ofthe ingredients as follows:

Diphenylmethylsodium:0.25×10⁻³ mol

Methyl methacrylate:5 g

N-cyclohexylmaleimide:5 g

Lithium chloride:0.25×10⁻² mol

Tetrahydrofuran:100 ml

Mn (methacrylate):18,300

Mn (maleimide):7,700

Mw/Mn (total copolymer):1.10.

EXAMPLE 8 (COMPARATIVE)

1 g of N-cyclohexylmaleimide was polymerized in accordance with theprocedure of Example 1, but replacing sec-butyllithium by 0.27×10⁻³ molof cumylpotassium. The polymer obtained under these conditions, in ayield of 98%, had a number-average molecular weight of 6,000 and apolydispersity of 1.8.

EXAMPLE 9 (COMPARATIVE)

4 g of N-cyclohexylmaleimide were polymerized in accordance with theprocedure of Example 1, but replacing sec-butyllithium by 0.85×10⁻³ molof tert-butyllithium. The polymer obtained under these conditions, in ayield of 100%, had a number-average molecular weight of 3,900 and apolydispersity of 1.8.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed:
 1. A block copolymer comprising at least one block ofan N-substituted maleimide homopolymer having a number average molecularweight ranging from 12,000 to 100,000.
 2. A block copolymer comprisingat least one block of an N-substituted maleimide homopolymer having apolydispersity index ranging from 1.1 to 1.5.
 3. A block copolymercomprising at least one block of an N-substituted maleimide homopolymerhaving a number-average molecular weight ranging from 12,000 to 100,000and having a polydispersity index ranging from 1.1 to 1.5.
 4. The blockcopolymer as defined by claim 1, comprising at least one block of anacrylic or methacrylic homopolymer.
 5. The block copolymer as defined byclaim 2, comprising at least one block of an acrylic or methacrylichomopolymer.
 6. The block copolymer as defined by claim 3, comprising atleast one block of an acrylic or methacrylic homopolymer.
 7. The blockcopolymer as defined by claim 4, said at least one acrylic ormethacrylic block having a number-average molecular weight ranging from2,000 to 100,000.
 8. The block copolymer as defined by claim 5, said atleast one acrylic or methacrylic block having a number-average molecularweight ranging from 2,000 to 100,000.
 9. The block copolymer as definedby claim 6, said at least one acrylic or methacrylic block having anumber-average molecular weight ranging from 2,000 to 100,000.
 10. Theblock copolymer as defined by claim 4, comprising at least one endblockof said N-substituted maleimide homopolymer.
 11. The block copolymer asdefined by claim 5, comprising at least one endblock of saidN-substituted maleimide homopolymer.
 12. The block copolymer as definedby claim 6, comprising at least one endblock of said N-substitutedmaleimide homopolymer.
 13. The block copolymer as defined by claim 4,comprising at least one endblock of said acrylic or methacrylichomopolymer.
 14. The block copolymer as defined by claim 5, comprisingat least one endblock of said acrylic or methacrylic homopolymer. 15.The block copolymer as defined by claim 6, comprising at least oneendblock of said acrylic or methacrylic homopolymer.